The quality of the paper resulting from a paper manufacturing process depends on many parameters associated with the pulp. In the pulp making industry there is a general shortage of reliable measurement techniques for characterisation of the pulp. Futhermore, true on-line techniques are even more rare. Thus, the control and quality assessment of the processes can not be accomplished in an efficient manner.
The refining process step in a mechanical pulping process is one of the most important steps of the entire paper making process. The way to operate the refining determines many of the most important properties of the resulting pulp. It is therefore desirable to be able to control the result from the refining process very carefully, in order to adjust the operation of the refining.
Some basic types of measurement philosophies exist for measuring pulp properties; “off-line”, “on-line”, “at-line” and “in-line”.
The classical off-line procedure is to extract samples of the pulp for analysis in a laboratory. However, in this way only a part of the pulp is analysed, and the possible feedback of such an analysis is generally slow. An analysis method suitable for providing data for control purposes has to be performed in direct contact with the actual pulp flow.
To speed up the off-line procedure (up to 5-10 times) an on-line procedure with automatic sampling systems have been developed in which measurements based on e.g. optical measurement techniques are used. Typically such systems operate by diverting a small portion of the pulp into a special pipe or volume. One example being the PQM system (Pulp Quality Monitor) from Sunds Defibrator, which measures freeness, fibre length and shive content in a pulp suspension.
A common problem with all off-line and some on-line and at-line methods is that only a part of the flow is measured. The properties in such a diversion flow may differ from the main flow. TCA (Thermomechanical pulp Consistency Analyser) from ABB AB measures the consistency of the pulp. The system is using fibre optic techniques. Other similar systems are the Smart Pulp Platform (SPFM) available from ABB AB, and “Fiber Master” developed by the Swedish pulp and paper research institute (SITFI).
In-line methods, which operates directly on the entire pulp without extracting fluid into a special test space, are generally faster than off-line methods and can reduce some of the problems listed for these methods. However, mechanical devices have to be inserted in the process line in order to extract the flow sample, which may disturb the main flow and which makes maintenance or replacement work difficult. Furthermore sensors may be contaminated, or the flow may be contaminated by the sensors.
An alternative to use optical or electromagnetic waves is to use mechanical (acoustical) waves. This has several advantages. Acoustic waves are enviromentally friendly and also unlike electromagnetic waves they can propagate in all types of fluids.
In the article “Ultrasonic propagation in paper fibre suspensions” by D. J. Adams, 3rd International IFAC Conference on Instrumentation and Automation in the Paper, Rubber and Plastics Industries, p. 187-194, Noordnederlands Boekbedrijf, Antwerp, Belgium, it is disclosed to send ultrasonic beams of frequencies between 0.6 MHz and 15 MHz through a suspension of fibres and the attenuation as well as the phase velocity can be measured as a function of frequency. It is by this possible to obtain information about fibre concentration, size and to some extent the fibre state. However, an elaborate calibration procedure is necessary in order to make the method operable.
In “Pulp suspension flow measurement using ultrasonics and correlation” by M. Karras, E. Harkonen, J. Tomberg and O. Hirsimald, 1982 Ultrasonics Symposium Proceedings, p. 915-918, vol. 2, Ed: B. R. MCAvoy, IEEE, New York, N.Y., USA, a transit time measurement system is disclosed. The system measures primarily the mean flow velocity and tests from various pulp suspensions are described. Doppler shift measurements are used to determine velocity profiles. A frequency of 2.5 MHz was used.
In the international patent application WO99/15890, a method and a device for pulp process monitoring using acoustic measurements were disclosed. Inherent acoustical fields in the system (up to 100 kHz) are recorded indirectly via wall vibration measurements on a conveyor line, through which a pulp suspension flows. The recordings are graded by a data manipulation program according to predetermined characteristics and a vibration characteristics is generated. Stored vibration characteristics related to earlier recordings are compared at each recording for correlation to the properties of the suspension. The recorded vibrations can be used for controlling the process in a suitable way, for raising alarms at fault situations or for showing changed tendencies.
Since the method used in the above patent is based on a method which makes use of inherently appearing vibrations a number of problems result. One being that not only will sound generated in the fluid be picked up but also vibrations from mechanical sources, e.g., pumps, connected to the fluid. This leads to large amounts of disturbances, which increases the amount of averaging or overdetermination. Futhermore, since there are no control of the source, process methods for suppressing disturbances are difficult to apply. In addition the suggested method must be calibrated for each individual site, since the inherent vibrations are site dependent. This last aspect is a considerable practical limitation since it will cause very large losses in production upon installation.